In manufacturing certain rubber articles, it is necessary to adhere pre-cured rubber components to uncured (green) rubber components. For instance, in retreading tires, an uncured tread can be affixed to a previously cured tire carcass. On the other hand, a previously cured tread could be affixed to an uncured carcass to simplify the process of building a new tire. In still other cases, it is desirable to adhere two pre-cured rubber components together.
Attaining adequate adhesion between a pre-cured rubber component and an uncured rubber component frequently proves to be challenging. An even greater challenge is presented in cases where it is desired to affix two pre-cured rubber components together. In such cases, cements and surface activation are frequently used to promote better adhesion. For example, surface activation is usually performed by buffing the surface of the pre-cured rubber component. However, the level of adhesion that can be attained via surface activation and cements sometimes proves to be inadequate.
This invention relates to a process for improving the adhesion between different rubber components in a rubber article, such as a tire, a hose or a belt. This technique can be used to improve the adhesion between two different pre-cured rubber components or for improving the adhesion between a pre-cured rubber component and an uncured rubber component. However, the technique of this invention can also be used to improve the adhesion between two uncured rubber components.
The technique of this invention is based upon the unexpected discovery that low molecular weight trans-1,4-polybutadiene containing rubber compounds can be used to improve the adhesion between rubber components in a rubber article. The adhesion-promoting rubber compositions of this invention will normally contain from about 10 weight percent to about 40 weight percent low molecular weight trans-1,4-polybutadiene and from about 60 weight percent to about 90 weight percent of at least one rubbery polymer. The rubbery polymer employed in such blends will typically be natural rubber or styrene-butadiene rubber. However, various other rubbery polymers (such as, synthetic polyisoprene, isoprene-butadiene rubber, styrene-isoprene-butadiene rubber or cis-1,4-polybutadiene rubber) can be utilized in the adhesion-promoting rubber composition. It is critical for the trans-1,4-polybutadiene employed in such blends to have a low molecular weight of no greater than about 150,000 and a trans-microstructure content which is within the range of about 60 percent to about 90 percent. It is preferred for the trans-1,4-polybutadiene to have a molecular weight of less than about 120,000.
The present invention more specifically discloses an adhesion-promoting rubber composition which is comprised of (a) from about 10 weight percent to about 40 weight percent trans-1,4-polybutadiene, wherein said trans-1,4-polybutadiene has a number average molecular weight which less than about 150,000 and wherein said trans-1,4-polybutadiene has a trans-microstructure content which is within the range of about 60 percent to about 90 percent and (b) from about 60 weight percent to about 90 weight percent of at least one rubbery polymer.
The subject invention further reveals a technique for improving the adhesion between a first rubber component and a second rubber component in a process for manufacturing a cured rubber article, said technique comprising the steps of: (1) positioning a layer of an adhesion-promoting rubber composition which is comprised of a low molecular weight trans-1,4-polybutadiene rubber between the first rubber component and the second rubber component, (2) bringing the first rubber component into contact with one side of the layer of adhesion-promoting rubber composition and bringing the second rubber composition into contact with the other side of the layer of adhesion-promoting rubber composition and (3) curing the first rubber composition, the second rubber composition and the adhesion-promoting rubber composition together under conditions of heat and pressure to produce the cured rubber article.
The trans-1,4-polybutadiene used in the adhesion promoting rubber compositions of this invention is a thermoplastic resin by virtue of its high level of crystallinity. Because trans-1,4-polybutadiene (TPBD) contains many double bonds in its backbone, it can be blended and cocured with rubbers. Even though TPBD is a thermoplastic resin, it becomes elastomeric when cured alone or when cocured with one or more rubbers. The TPBD used in manufacturing the rubber articles of this invention typically has a trans-microstructure content which is within the range of about 60 percent to about 90 percent and a number average molecular weight which is within the range of about 5,000 to about 150,000.
The TPBD will preferably have a trans-microstructure content which is within the range of about 75 percent to about 85 percent. It will more preferably have a trans-microstructure content which is within the range of about 78 percent to 82 percent. The TPBD will preferably have a number average molecular weight which is within the range of about 50,000 to about 120,000 and will most preferably have a number average molecular weight which is within the range of about 70,000 to about 100,000. Such TPBD will typically have a Mooney ML-4 viscosity at 100xc2x0 C. which is within the range of about 5 to about 20. The trans-1,4-polybutadiene will typically have a melting point which is within the range of about 100xc2x0 C. to about 30xc2x0 C. It will also have a glass transition temperature which is within the range of about xe2x88x92100xc2x0 C. to about xe2x88x9280xc2x0 C.
TPBD which is suitable for use in making the rubber articles of this invention can be made by various polymerization techniques. For instance, the TPBD can be synthesized by utilizing the procedure described in U.S. Pat. No. 5,753,579. The teachings of U.S. Pat. No. 5,753,579 are incorporated herein by reference in their entirety. In this technique, 1,3-butadiene monomer is polymerized into the TPBD in the presence of a barium catalyst system which is comprised of at least one organolithium initiator, an organoaluminum compound, a barium alkoxide and an organozinc compound.
The TPBD can be synthesized by (1) mixing (a) an organolithium initiator or an organomagnesium initiator, (b) an organoaluminum compound, (c) a barium alkoxide and (d) an organozinc compound to produce a preformed initiator system; and (2) adding the preformed initiator system to a polymerization medium which is comprised of an organic solvent and 1,3-butadiene monomer. In such polymerizations, normally from 0.01 to 1 phm (parts per 100 parts by weight of monomer) of an organolithium initiator will be utilized. The molar ratio of the organoaluminum compound to the barium alkoxide will typically be within the range of about 0.1:1 to about 8:1. The molar ratio of the organolithium compound to the barium alkoxide will normally be within the range of about 0.1:1 to about 8:1. The molar ratio of the organozinc compound to the barium alkoxide will typically be within the range of about 0.1:1 to about 8:1. The polymerization temperature utilized will normally be within the range of about 40xc2x0 C. to about 120xc2x0 C.
The TPBD can also be synthesized with a cobalt-based catalyst systems. For example, U.S. Pat. No. 5,089,574 reveals a process for synthesizing trans-1,4-polybutadiene in a continuous process which comprises continuously charging 1,3-butadiene monomer, an organocobalt compound, an organoaluminum compound, a para-substituted phenol, carbon disulfide and an organic solvent into a reaction zone; allowing the 1,3-butadiene monomer to polymerize in said reaction zone to form the trans-1,4-polybutadiene; and continuously withdrawing the trans-1,4-polybutadiene from said reaction zone.
U.S. Pat. No. 5,448,002 discloses that dialkyl sulfoxides, diaryl sulfoxides and dialkaryl sulfoxides act as molecular weight regulators when utilized in conjunction with cobalt-based catalyst systems in the polymerization of 1,3-butadiene monomer into TPBD. U.S. Pat. No. 5,448,002 reports that the molecular weight of the TPBD produced decreases with increasing levels of the dialkyl sulfoxide, diaryl sulfoxide or dialkaryl sulfoxide present as a molecular weight regulator.
The molecular weight of TPBD synthesized with cobalt-based catalyst systems can be reduced so as to be within the desired molecular weight range by depolymerizing the TPBD with a methasis catalyst. For example, the molecular weight of the TPBD can be reduced with tungsten hexachloride/triisobutyl aluminum/ethanol catalysts.
The adhesion-promoting rubber compositions of this invention are made by simply mixing the TPBD with one or more rubbery polymers. A wide variety of rubbery polymers can be used for this purpose. Some representative examples of rubbery polymers that can be used include natural rubber, styrene-butadiene rubber, synthetic polyisoprene, isoprene-butadiene rubber, styrene-isoprene-butadiene rubber and cis-1,4-polybutadiene. A blend of one or more these rubbery polymers can, of course, be utilized. Normally natural rubber and styrene-butadiene rubber are preferred with natural rubber being most preferred.
The adhesion-promoting rubber composition will typically contain from about 10 weight percent to about 40 weight percent of the TPBD and from about 60 weight percent to about 90 weight percent of the rubbery polymer. It is normally preferred for the adhesion-promoting rubber composition to contain from about 15 weight percent to about 30 weight percent of the TPBD and from about 70 weight percent to about 85 weight percent of the rubbery polymer. It is generally more preferred for the adhesion-promoting rubber composition to contain from about 18 weight percent to about 22 weight percent of the TPBD and from about 78 weight percent to about 82 weight percent of the rubbery polymer.
The adhesion-promoting rubber composition will normally be extruded or milled into a relatively thin layer for affixing the rubber components together. The layer of adhesion-promoting rubber composition will typically have a thickness which is within the range of about 10 mils (0.25 mm) to about 300 mils (7.62 mm). The layer of adhesion-promoting rubber composition will preferably have a thickness that is within the range of about 50 mils (1.27 mm) to about 160 mils (4.064 mm) and will more preferably have a thickness which is within the range of about 80 mils (2.03 mm) to about 120 mils (3.05 mm).
The technique of this invention simply involves positioning the layer of adhesion-promoting rubber composition between the two different rubber components being bound together. For instance, a layer of the adhesion-promoting rubber composition can be employed as a cushion in retreading tires. Then, the rubber components are forced together by the application of pressure with the adhesion-promoting rubber composition being trapped between the two rubber components. Strong adhesion can then be attained by curing (vulcanizing) the rubbers utilizing standard procedures. In most cases, the vulcanization will be carried out at a temperature which is within the range of about 100xc2x0 C. to about 300xc2x0 C. However, it is normally preferred to utilize a curing temperature that is within the range of about 135xc2x0 C. to about 175xc2x0 C. The curing step will, of course, also normally be carried out while applying pressure.